Well, my wife just went to Dunn tire yesterday to get front tires and they gave her a brochure about how they are using nitrogen now instead of air...didn`t get to read it tho. I hope her life insurance is paid up. Ka-CHING!

I think the biggest reason it's used, which Wingnut eluded too, is Nitrogen's stablity throughout temperature extremes. It pretty gets cold at high altitudes (especially space) and the air in the tires would contract resulting in a lower tire pressure. Given the speed with which aircraft and particularly the shuttle go from extremes of cold to warm it would not give the air time to warm up thus resulting in the aircraft essentially landing with a flat tire. Not a good situation for anyone. I guess it's probably the opposite for race cars. The tires get pretty hot from all the friction and they don't want to air to expand too much causing a blowout from too much air pressure.
I know you all probably didn't need that big of an explaination but hey...when I know something I have to show off as it doesn't happen that often